Astrophotography Mounts: Periodic Error Correction

Introduction

This article is a discussion of Periodic Error and its correction with a mount feature called (surprisingly) Periodic Error Correction. This feature applies only to moderate- and high-quality motorized mounts when they are used for long-exposure astrophotography. We provide a brief explanation of this error and of a common mount feature that that is offered for reducing it.

What is Periodic Error?

Periodic Error is a small mechanical error in the accuracy of the tracking in a motorized mount resulting in small movements of the target that can spoil long-exposure images, even if the mount is perfectly polar-aligned and appears to be tracking perfectly in short tests. It is called periodic because it repeats at a regular interval — the interval being the amount of time it takes the mount’s drive gear to complete one revolution. Most motorized mounts suitable for astrophotography use moderate-speed motors and a gearing system to produce the very smooth, slow-speed rotation of the Right Ascension axis needed to track target objects.

On most such mounts, the drive motor turns a worm gear which, in turn, drives a large disk gear attached to the Right Ascension axis. These gears are machined to a very high precision. However, no machining is perfect, and the manufacturing is also constrained by the overall price target for the class of mount. So, minor imperfections will always remain in the gears.

Imagine, for example, if the teeth on the worm gear were displaced slightly to one side at one point on the gear. When that point on the worm rotated through the RA gear, the RA would be pushed either too far ahead, or too far behind, depending on the direction of the error. The error might self-correct if the next portion of the worm gear is distorted in the other direction, or it might accumulate.The error would repeat the next time, and every time, that flawed bit of worm gear rotated around to contact the RA gear. This error that repeats with each revolution of the worm gear is Periodic Error.

This simulation is exaggerated — typically the error would cycle over a period of several minutes.

(This animation is an “animated gif” graphic. If you are not seeing moving pictures, make sure that your browser settings have not disabled animated gif graphics. Better still, click on the image to view it stand-alone in a separate window.)

Periodic Error is small back-and-forth oscillations of the target stars, along the East-West line. The small motion is often not noticeable during visual observation, but during a long exposure it will change stars from pinpoints to small streaks.If your stars are small streaks and the streaks a perfectly aligned east-west, you probably have periodic error.

(If the streaks are aligned in some other direction, or are curved, you probably have inadequate polar alignment.)

Reducing Periodic Error

Periodic Error can be reduced to an acceptable level using a variety of techniques, only some of which are in the range of a beginner or mid-level astrophotographer.

Throw Money at the Problem

Very high-end mounts for astrophotography have very small periodic error because of the time and money spent on manufacturing high-precision gears. You can also buy higher-precision gear upgrades for some mounts.

For the beginner, let’s call this impractical. Buying a $1000 replacement worm gear for your $500 mount is probably not a good balance.

Throw Even More Money at the Problem

If you still don’t feel you’ve spent enough money, many very high-end mounts can be equipped with Absolute Encoders, which means the mount always knows precisely where it is pointing. This essentially eliminates periodic error. Again, let’s assume that this $10,000 option is not of immediate interest to most beginners.

Don’t Use Gears

This is another case of the “throw money” solution, mentioned just for fun. There are some very-high-end experimental drive systems showing up on the market that don’t use worm gears, and that don’t exhibit periodic error. Examples include direct drive systems and harmonic drives. ‘Way out of our price range.

Autoguiding

A second guide camera and computer can be used to make frequent small corrections to the mount’s pointing, and this can reduce or eliminate Periodic Error if the onset of the error is not too sudden. This is the subject of a separate article.

Periodic Error Correction Feature

Most mounts intended for astrophotography include a feature called Periodic Error Correction (PEC), which can be used alone, or in conjunction with Autoguiding. PEC is used in two phases:

1. Training. During this phase you use the control panel or menus to say “Hey! Pay attention to this!” to the mount, then you manually keep a star perfectly centred for several worm periods. You do this by centring a star with a high-magnification eyepiece that includes a cross-hair reticle. You then stare at the star for 10 to 15 minutes and use the mount’s hand controller to manually make the small adjustments necessary to keep it perfectly centred on the cross hair. The mount records the error corrections you supply, remembering where in the worm position each one was needed. By training through more than one worm cycle, the mount can record an average correction, in case you reacted slowly or over-reacted.

   Understand, training your PEC is something you do before you take a photo, not while you are taking a photo. Typically you don't have to train PEC very often - just when the seasons change or something has physically affected your mount.

2. Playback. Once you have trained the mount, you can turn on PEC. The mount will “play back” the recorded error correction information by slightly changing the speed of the Right Ascension drive to move ahead or back each time your manual error corrections did the same. This will cancel the periodic error, resulting in a smoother track.

The manual training phase was horribly tedious and no one does that any more. A more modern alternative is to use a camera and computer — usually the same one you will use for auto-guiding — to track a star while the mount records the training information.

There is even specialized software available to help collect Periodic Error data, analyze and smooth it, and upload it to the mount. You do not need such software, as the manual techniques mentioned above will work just fine. However, it makes a tedious job simple and pleasant, and you may find it a worthwhile investment. As a beginner I used PEMPRO and was very impressed with it. It’s not free, but it is inexpensive and works very well (and includes another feature to help achieve perfect polar alignment).